This is one KPI, now is the time to look at it seriously and that is the Year on Year sales growth to determine how you have been going. It is called YOY growth.
The formula generally is listed as compared to last year, so it is ((current year sales)/ (last year sales)-1) x 100, which is actually what most people outside your organisation expect to see.
One big plus with doing it year by year is that it removes the effect that monthly figures often have eg 4 Saturdays and Sundays this year in a month and 5 Saturdays and Sundays in last year's month.
I would also suggest doing it by departments then with a grand total.
If you handle agency items, eg dry cleaning, lotto, etc. then it is the commissions that you measure not the turnover as the turnover is not considered your money.
Your year-over-year calculations can help you measure your business’s performance. You can see if your business is growing from year to year, not just month to month. You can easily see long-term trends and if your business is improving over time.
Note in my experience before you start to worry about dramatic changes check the options you used in your POS software, far too often people have used the wrong dates.